1. Field of Invention
The present invention relates to a light guide plate emitting light from an emission face after applying direction-conversion within the light guide plate to light supplied thereto sideways, and to a surface light source device and display employing the light guide plate.
2. Related Art
A well-known technique gives a surface light source device that employs a light guide plate having a side part for light introduction and an emission face provided by one of two major faces (i.e. faces larger than end faces), the surface light source device being applied to various uses such as back-lighting for a liquid crystal display. A surface light source device of such a type has a basic performance greatly depending on a light guide plate employed therein.
A basic function of a light guide plate is to change a propagation direction (roughly in parallel with an emission face of the light guide plate) of light introduced into the light guide plate through a side end face so that the light is emitted through the emission face. As known well, a simply transparent light guide plate without any modification is capable of deflecting light little, providing a unsatisfactory brightness.
Therefore any means for promoting emission through the emission face is required. The means is preferably capable of providing a sufficient illumination output directed to a desired direction (usually, a generally frontal direction) without a particular need of light propagation direction modifying member (such as prism sheet).
It is noted that a light propagation direction modifying member, typically a prism sheet, is used often, as known well, in order to modify a greatly forward-inclined direction of an emission from an emission face and to obtain an illumination output directed to a desired direction (usually, a generally frontal direction).
Under such background, the present inventor has proposed a light guide plate provided with so-called micro-reflectors and surface light source device/LCD employing the light guide plate, being disclosed in Japanese Patent Application Tokugan-Hei 11-38977 and PCT/JP00-00871 (WO00-49132).
According to this proposed prior art, a light guide plate has a back face provided with a great number of micro-reflectors. FIG. 11 exemplarily shows one of them to illustrates an emission promotion effect. As shown in FIG. 11, each micro-reflector has first and second reflection surfaces (called inner-reflection surfaces, hereafter) which provide a pair of inclined reflection surfaces forming a valley inside.
Upon reaching a micro-reflector and entering into a valley thereof on the way of propagation within the light guide plate, light is direction-converted mainly by inner-reflections effected twice. Such double inner reflections consist of one inner reflection effected at one of the first and second reflection surfaces and the other inner reflection effected at the other of the first and second reflection surfaces.
This direction conversion produces a light directed to the emission face, promoting omission from the emission face. A propagation direction of light after being direction-converted by each micro-reflector mainly depends on inclinations (i.e. spatial directions) of the first and second reflection surfaces of the micro reflector. Therefore angular emission characteristics of the light guide plate as a whole can be controlled within a remarkable range by means of adjusting factors such as direction distribution of reflection surfaces, or arraying pattern or distribution of the micro reflectors.
According to typical designs, it is aimed that a light guide plate allows an emission face of the light guide plate to emit the most intensive light toward a generally frontal direction. Employable primary light sources for supplying light to a light guide plate include a line-like light source such as cold cathode lamp and point-like light source such as light emitting diode, both being usable for providing a surface light source device to have an improved brightness.
However, a problem arises if the above-proposed prior art is applied to a case where a plurality of point-like light sources are adopted as primary light source, a problem arises. The problem is simply described with referring to FIG. 12. FIG. 12 illustrating an example of arrangement employing a plurality of point-like light sources as proposed in the above prior art.
This arrangement comprises point-like light sources (light emitting diodes) disposed in dents (input portions), which are located at two positions on one side, and a great number of micro-reflector formed on a back face. Each micro-reflector is shaped as shown in FIG. 11 and has an orientation represented by a ridge formed by first and second reflection surfaces meeting each other. The orientation in this arrangement is as follows.
About half of all of the micro-reflectors have ridges orientated as to directed to one of the point-like light sources (input portions). It is noted that such xe2x80x9cabout half of all the micro-reflectorsxe2x80x9d are distributed all over the back face. On the other hand, the other about half of all of the micro-reflectors have ridges orientated as to directed to the other of the point-like light sources (input portions) It is noted that such xe2x80x9cthe other about half of all of the micro-reflectorsxe2x80x9d are also distributed all over the back face.
Therefore, some areas have micro-reflectors provided with very irrational orientations. For example, although some micro-reflectors are much nearer to xe2x80x9cone point-like light sourcexe2x80x9d than to xe2x80x9cthe point-like light sourcexe2x80x9d, many of them are focused to xe2x80x9cthe other point-like light sourcexe2x80x9d, with the result that said many micro-reflectors receive a strong light sideways. This brings an inefficient direction-conversion and a direction-conversion into an undesirable direction, possibly affecting emission of inner propagation light.
An object of the present invention is to provide at light guide plate suitable for introducing light from a plurality of input portions using point-like light sources, a surface light source device and a display employing the light guide plate. Viewing from another angle, the present invention aims to provide a light guide plate and a surface light source device which are capable of outputting an illumination light at a high efficiency and to realize a bright display by applying them to a display.
In the first place, the present invention is applied to a light guide plate having a plurality of input portions through which an illumination light emitted from a point-like light source is introduced and an emission face provided by a major face through which the illumination light introduced is emitted on the way of inner propagation.
According to an improvement provided by the present invention, the light guide plate has a back face opposite with the emission face and divided into a plurality of regions to each of which any of the input portions is allotted, each of the regions being provided with a great number of micro-reflectors for converting a direction of an inner propagation light into a direction toward the emission face by effecting an inner reflection. And the micro-reflectors located in each of the regions are orientated toward an input portion allotted to the each region.
This enables the micro-reflectors to have orientations determined in consideration for situation of each region provided by the above division. In other words, micro-reflector orientations can be determined, for each region, depending on which input portion should be aimed by micro-reflectors in each region. As a result, an efficient direction conversion into a desirable direction (typically, a generally frontal direction) is effected by the micro-reflectors.
A micro-reflector may be shaped like a quadrangle pyramid having a pair of inner-reflection slopes which meet each other as to provide a ridge representing an orientation of the micro-reflector. The ridge is directed toward the input portion allotted and gets closer to the back face with an increasing distance from the input portion allotted.
Alternatively, a micro-reflector may be shaped like a cylindrical concavity having an inner-reflection curved surface which represents an orientation of the micro-reflector. The inner-reflection curved surface is directed toward the input portion allotted. Further alternatively, a micro-reflector may be shaped like a V-shaped concavity having an inner-reflection slope which represents an orientation of the each micro-reflector. The inner-reflection slope is directed toward the input portion allotted.
According to another improvement provided by the present invention, the light guide plate has a back face opposite with the emission face and divided into a plurality of regions to each of which any of the input portions is allotted and each of the regions is provided with a great number of micro-reflectors. They include a first group of micro-reflectors orientated toward an input portion allotted to the each region and a second group of micro-reflectors which are intermingled with the first group of micro-reflectors and orientated toward another input portion allotted to different region adjacent to the each region.
And the second group of micro-reflectors have a covering rate the ratio of which to that of the first group of micro-reflectors gets greater with an decreasing distance from the adjacent region.
This enables the micro-reflectors to have orientations determined in consideration for not only situation of each region provided by the above division but also situation around boundaries among the respective regions. That is, a more efficient and uniform emission promotion is realized because of avoiding affection which could be caused by a sharp changing of orientation state around the boundaries among the respective regions.
A micro-reflector may be shaped like a quadrangle pyramid having a pair of inner-reflection slopes which meet each other as to provide a ridge representing an orientation of the micro-reflector. Each ridge of each of the first group of micro-reflectors is directed toward the input portion allotted and gets closer to the back face with an increasing distance from the input portion allotted while each ridge of each of the second group of micro-reflectors is directed toward the another input portion and gets closer to the back face with an increasing distance from the another input portion.
Alternatively, a micro-reflector may be shaped like a cylindrical concavity having an inner-reflection curved surface which represents an orientation of the each micro-reflector. Each inner-reflection curved surface of each of the first group of micro-reflectors is directed toward the input portion allotted while each inner-reflection curved surface of each of the second group of micro-reflectors is directed toward the another input portion.
Further alternatively, a micro-reflector may be shaped like a V-shaped concavity having an inner-reflection slope which represents an orientation of the each micro-reflector. Each inner-reflection slope of each of the first group of micro-reflectors is directed toward the input portion allotted while each inner reflection slope of each of the second group of micro-reflectors is directed toward the another input portion
In the next place, the present invention is applied to a surface light source device comprising a light guide plate and point like light sources arranged corresponding to input portions of the light guide plate respectively. Further the present invention is applied to a display having a liquid crystal display panel and a surface light source device for illuminating the liquid crystal display panel. According to features of the present invention, the light guide plate is improved according to any of the above improvements. Needless to say, merits of such an improved light guide plate is maintained in the surface light source device and display.
That is, without a particular need of light propagation direction modifying member such as prism sheet, the surface light source device is capable of providing a sufficient illumination output directed to a desired direction (usually, a generally frontal direction and the display can have a liquid crystal display panel illuminated efficiently by the surface light source device.